Cohesion

Definition

Cohesion refers to the attractive forces that exist between like molecules, causing them to cling together and form a more organized and stable structure. This concept is particularly relevant in the context of intermolecular forces, which govern the interactions between molecules and shape the physical properties of substances.

5 Must Know Facts For Your Next Test

1. Cohesion is a crucial factor in determining the physical properties of a substance, such as its boiling point, surface tension, and viscosity.
2. The strength of cohesive forces is directly related to the polarity of the molecules involved, with more polar molecules exhibiting stronger cohesive interactions.
3. Hydrogen bonding is a particularly strong type of intermolecular force that contributes significantly to the cohesion of water molecules, leading to water's unique properties.
4. Cohesion is responsible for the formation of surface tension in liquids, which allows certain insects and plants to walk on water or transport water through their vascular systems.
5. The cohesive forces between molecules in a solid state are typically stronger than the cohesive forces in a liquid state, leading to the more organized and rigid structure of solids.

Review Questions

• Explain how the concept of cohesion relates to the physical properties of a substance.
  • The cohesive forces between like molecules directly influence the physical properties of a substance. Stronger cohesive forces, such as those found in water due to hydrogen bonding, lead to higher boiling points, surface tension, and viscosity. Conversely, weaker cohesive forces, as seen in nonpolar molecules, result in lower boiling points and less viscous liquids. The degree of cohesion within a substance is a key factor in determining its macroscopic behavior and characteristics.
• Describe the role of polarity in determining the strength of cohesive forces between molecules.
  • The polarity of molecules is a crucial factor in determining the strength of cohesive forces. Polar molecules, which have an uneven distribution of electrons and a resulting partial positive and partial negative charge, exhibit stronger cohesive interactions due to the attraction between the partially charged regions of neighboring molecules. This is exemplified by the strong hydrogen bonding that occurs between water molecules, leading to water's high boiling point and surface tension. In contrast, nonpolar molecules with a more even electron distribution experience weaker cohesive forces, resulting in different physical properties.
• Analyze how the concept of cohesion can be applied to understand the behavior of substances in different states of matter.
  • The concept of cohesion can be used to explain the differences in the behavior of substances in different states of matter. In the solid state, the cohesive forces between molecules are typically stronger, leading to a more organized and rigid structure. As a substance transitions to the liquid state, the cohesive forces become weaker, allowing the molecules to move more freely and resulting in a less structured arrangement. Further, the cohesive forces in the liquid state are still strong enough to maintain the integrity of the substance and contribute to properties like surface tension. In the gaseous state, the cohesive forces between molecules are significantly weaker, allowing the molecules to move independently and occupy a larger volume. By understanding the role of cohesion, one can better predict and explain the physical properties and behavior of substances in their various states of matter.

Related terms

Intermolecular Forces:

Intermolecular forces are the attractive and repulsive forces that exist between molecules, influencing their behavior and the properties of the substance they form.

Hydrogen Bonding:

Hydrogen bonding is a specific type of intermolecular force that occurs when a hydrogen atom covalently bonded to a highly electronegative element, such as oxygen or nitrogen, interacts with another highly electronegative element.

Dipole-Dipole Interactions:

Dipole-dipole interactions are intermolecular forces that arise between polar molecules, where the partially positive end of one molecule is attracted to the partially negative end of another molecule.